Nippon Steel & Sumitomo Metal Corporation

Nippon Steel Corporation

Sumitomo Metal Industries, Inc.

Toshiro Tomida

Yasuaki Tanaka

Takuya Nishio

Kaori Kawano

Jun Haga

Norio Imai

Kengo Hata

Yoshiaki Nakazawa

Masayuki Wakita

Masahito Tasaka

Shigeo Uenoya

Mitsuru Yoshida

Hiroshi Takebayashi

Suguhiro Fukushima

Naoyuki Sano

A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 &#x3bc;m. A number density of retained austenite grains having a grain size of 1.2 &#x3bc;m or more is 3.0&#xd7;10/&#x3bc;mor less.

Hot-dip galvanized cold-rolled steel sheet and process for producing same

A high-strength steel sheet has a chemical composition comprising C: 0.05-0.20%, Si: 0.02-3.0%, Mn: 0.5-3.0%, P: at most 0.5%, S: at most 0.05%, Cr: 0.05-1.0%, sol. Al: 0.01-1.0%, one or more elements selected from the group consisting of Ti, Nb, Mo, V, and W: a total of 0.002-0.03%, and a remainder of Fe and impurities. The sheet has an average grain diameter of ferrite of at most 3.0 &#x3bc;m at least in a region of 100-200 &#x3bc;m in the sheet thickness direction from the surface of the steel sheet. The average spacing in the sheet thickness direction of the remaining structure in this region is at most 3.0 &#x3bc;m. Mechanical properties include at least 750 MPa tensile strength and at least 13,000 MPa&#xb7;% (tensile strength&#xd7;elongation).

Steel sheet and a method for its manufacture

A high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability includes: a chemical composition consisting, in mass %, C: 0.06 to 0.3, Si: 0.6 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, total of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities; a microstructure having a main phase including at least 40 area % in total of martensite and/or bainite; and a texture in which proportion of an average X-ray intensity in an {100}&#x3c;011&#x3e; to {211}&#x3c;011&#x3e; orientations relative to an average X-ray intensity of a random structure not having a texture is less than 6.

Cold-rolled steel sheet and process for manufacturing same

A hot-rolled steel sheet having a composition comprising, in mass %, C: 0.06 to 0.3%, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to 1%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003%, the remainder Fe and impurities, is cold rolled, then heated at an average heating rate of at least 15&#xb0; C./sec so that a proportion of an unrecrystallization of a region not transformed to austenite at a time of reaching (Acpoint+10&#xb0; C.) is at least 30 area %, and then held for 30 seconds at temperature of at least (0.9&#xd7;Acpoint+0.1&#xd7;Acpoint) and at most (Acpoint+100&#xb0; C.).

Process for manufacturing cold-rolled steel sheet

A steel material comprising, by mass%, C: greater than 0.05% to 0.2%, Mn: 1% to 3%, Si: greater than 0.5% to 1.8%, Al: 0.01% to 0.5%, N: 0.001% to 0.015%, Ti or a sum of V and Ti: greater than 0.1% to 0.25%, Ti: 0.001% or more, Cr: 0% to 0.25%, Mo: 0% to 0.35%, the balance: Fe and impurities, comprising a multi-phase structure having a ferrite main phase and a second phase containing one or more of bainite, martensite and austenite, wherein an average nanohardness of the second phase is less than 6.0 GPa, an average grain diameter of all crystal grains in the main phase and the second phase is 3 &#x3bc;m or less, and a proportion of a length of small-angle grain boundaries where the misorientation is 2&#xb0; to less than 15&#xb0; in a length of all grain boundaries is 15% or more.

Steel material

A method for producing a high-tensile cold-rolled steel sheet includes subjecting a slab having a composition containing C: more than 0.020% and less than 0.30%, Si: more than 0.10% and 3.00% or less, and Mn: more than 1.00% and 3.50% or less to hot rolling wherein the roll draft of the final one pass is higher than 15%, and rolling is finished in the temperature region of Arpoint or higher, optionally annealing wherein the hot-rolled steel sheet is heated to 300&#xb0; C. or higher after being cooled to 780&#xb0; C. or lower, coiling higher than 400&#xb0; C. or lower than 400&#xb0; C., cold rolling the hot-rolled steel sheet or the annealed steel sheet, and annealing wherein the cold-rolled steel sheet is soaked in the temperature region of (Acpoint&#x2212;40&#xb0; C.) or higher, cooling to 500&#xb0; C. or lower and 300&#xb0; C. or higher, and holding in that temperature region for 30 seconds or longer.

Method for producing cold-rolled steel sheet

A multi-phase hot-rolled steel sheet having improved strength in an intermediate strain rate region has a chemical composition comprising, in mass percent, C: 0.07-0.2%, Si+Al: 0.3-1.5%, Mn: 1.0-3.0%, P: at most 0.02%, S: at most 0.005%, Cr: 0.1-0.5%, N: 0.001-0.008%, at least one of Ti: 0.002-0.05% and Nb: 0.002-0.05%, and a remainder of Fe and impurities. The area fraction of ferrite is 7-35%, the grain diameter of ferrite is in the range of 0.5-3.0 &#x3bc;m, and the nanohardness of ferrite is in the range of 3.5-4.5 GPa. A second phase which is the remainder other than ferrite contains martensite and bainitic ferrite and/or bainite. The average nanohardness of the second phase is 5-12 GPa, and the second phase contains a high-hardness phase of 8-12 GPa with an area fraction of 5-35% based on the overall structure.

Multi-phase hot-rolled steel sheet having improved dynamic strength and a method for its manufacture

A steel material contains: by mass %, C: greater than 0.05% to 0.18%; Mn: 1% to 3%; Si: greater than 0.5% to 1.8%; Al: 0.01% to 0.5%; N: 0.001% to 0.015%; one or both of V and Ti: 0.01% to 0.3% in total; Cr: 0% to 0.25%; Mo: 0% to 0.35%; a balance: Fe and impurities; and 80% or more of bainite by area %, and 5% or more in total of one or two or more selected from a group consisting of ferrite, martensite and austenite by area %, in which an average block size of the above-described bainite is less than 2.0 &#x3bc;m, an average grain diameter of all of the above-described ferrite, martensite and austenite is less than 1.0 &#x3bc;m, an average nanohardness of the above-described bainite is 4.0 GPa to 5.0 GPa, and MX-type carbides each having a circle-equivalent diameter of 10 nm or more exist with an average grain spacing of 300 nm or less therebetween.

A multi-phase hot-rolled steel sheet has a metallurgical structure having a main phase of ferrite with an average grain diameter of at most 3.0 &#x3bc;m and a second phase including at least one of martensite, bainite, and austenite. In the surface layer, the average grain diameter of the second phase is at most 2.0 &#x3bc;m, the difference (&#x394;nH) between the average nanohardness of the main phase (nH) and the average nanohardness of the second phase (nH) is 6.0-10.0 GPa, the difference (&#x394;&#x3c3;nH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the main phase is at most 1.5 GPa, and in the central portion, the difference (&#x394;nH) between the average nanohardnesses is at least 3.5 GPa to at most 6.0 GPa and the difference (&#x394;&#x3c3;nH) between the standard deviations of the nanohardnesses is at least 1.5 GPa.

Hot-rolled, cold rolled, and plated steel sheet having improved uniform and local ductility at a high strain rate

The steel material for an impact absorbing member has a composition containing: by mass %, C: 0.05 to 0.18%, Mn: 1 to 3%, Si+Al: at least 0.5% and less than 2.5%, and N: 0.001 to 0.015%, and in some cases, Cr: at most 0.5%, Mo: at most 0.2%, Ti: at most 0.05%, Nb: at most 0.05%, V: at most 0.2%, and B: at most 0.002%, the remainder being Fe and impurities. The steel material structure contains at least 70% by area of bainite made up of a lath structure having an average interval of at most 1 mm and martensite, 5 to 30%, and satisfies Formulas (1) and (2): (1) 1.2 &#xa3; H/H&#xa3; 1.6, (2) 0.90 &#xa3; {(H/H)/(H/H) &#xa3; 1.3, where Hand Hrepresent average nano hardness in an initial state and after 10% tensile deformation of the martensite, and Hand Hrepresent that of bainite, respectively.

Steel material and impact absorbing member

A steel sheet suitable as a starting material for a vehicle impact absorbing member with high absorption of impact energy and resistance to cracking contains, by mass %, C: 0.08-0.30%, Mn: 1.5-3.5%; Si+Al: 0.50-3.0%, P: 0.10% or less, S: at most 0.010%, and N: at most 0.010%, and optionally, one or more types selected from Cr: at most 0.5%, Mo: at most 0.5% , B: at most 0.010%, Ti: less than 0.04%, Nb: less than 0.030%, V: less than 0.5%, Ca: at most 0.010%, Mg: at most 0.010%, REM: at most 0.050%, and Bi: at most 0.050%. The microstructure contains, by area %, bainite: more than 50%, martensite: 3-30%, and retained austenite: 3-15%, the remainder comprising ferrite having an average grain diameter of less than 5 mm. The product of uniform elongation and hole expansion ratio is at least 300%and 5% effective flow stress is at least 900 MPa.

Steel sheet suitable for impact absorbing member and method for its manufacture

A high-strength cold-rolled steel sheet includes a composition having controlled amounts of carbon, silicon, manganese, phosphorous, sulfur, titanium, niobium, sol. Aluminum, chromium, molybdenum, vanadium, boron, calcium, REM, and iron. A microstructure thereof has a main phase of ferrite of at least 40 area %, and a second phase of a low-temperature transformation phase consisting either or both of martensite and bainite, which comprises at least 10 area % in total and retained austenite (&#x3b3;) at least comprising 3 area % . An average grain diameter of ferrite has a tilt angle of at least 15&#xb0; is at most 5.0 mm, an average grain diameter of the low-temperature transformation-produced phase is at most 2.0 mm, an average grain diameter of lump-like retained &#x3b3; having an aspect ratio of less than 5 is at most 1.5 mm, and an area fraction of the lump-like retained &#x3b3; relative to the retained &#x3b3; is at least 50%.

Cold-rolled steel sheet with controlled microstructure, grain diameters, and texture

A high-strength cold-rolled steel sheet excellent in ductility, work hardenability, and stretch flangeability, and having tensile strength of 780 MPa or more includes: a chemical composition containing, in mass percent, C: more than 0.020% to less than 0.30%, Si: more than 0.10% to 3.00% or less, Mn: more than 1.00% to 3.50% or less; and metallurgical structure whose main phase is a low-temperature transformation product, and whose secondary phase contains retained austenite. The retained austenite has a volume fraction relative to overall structure of more than 4.0% to less than 25.0% and an average grain size of less than 0.80 &#x3bc;m, and of the retained austenite, the number density of retained austenite grains whose grain size is 1.2 &#x3bc;m or more is 3.0&#xd7;10grains/&#x3bc;mor less.

Cold-rolled steel sheet

A cold-rolled steel sheet having a refined structure in which grain growth during annealing is suppressed has a chemical composition containing, in mass percent, controlled amounts of carbon, manganese, niobium, titanium, vanadium, sol. Aluminum, chromium, molybdenum, boron, calcium, and REM and a microstructure which contains at least 50% by area of ferrite as a main phase, a second phase containing at least 10% by area of a low temperature transformation phase and 0-3% by area of retained austenite and which satisfies the following Equations (1)-(3), in addition to a particular texture,&#x3c;2.7+10000/(5+300&#xd7;C+50&#xd7;Mn+4000&#xd7;Nb+2000&#xd7;Ti+400&#xd7;V)&#x2003;&#x2003;(1),&#x3c;4.0&#x2003;&#x2003;(2),and&#x2266;1.5&#x2003;&#x2003;(3),wherein dis the average grain diameter (&#x3bc;m) of ferrite defined by a high angle grain boundary having a tilt angle of at least 15&#xb0;, and dis the average grain diameter (&#x3bc;m) of the second phase.

Cold-rolled steel sheet and process for production thereof

(1) A doubly oriented silicon steel sheet having excellent magnetic characteristics in two directions, i.e. in a rolling direction and in a direction perpendicular to the rolling direction, and most suited for use as material for cores of small-sized transformers, and (2) a method for manufacturing the same. The doubly oriented silicon steel sheet as mentioned above in (1) is characterized in that: Si and Mn are contained in amounts which satisfy a predetermined formula of relation; an average crystal grain is as large as 1 to 8 times the thickness of the sheet as measured on a cross section parallel to the surface of the sheet; and at least 60% of all crystal grains have a size of X/3 to 3X, where X is an average grain size. In the doubly oriented silicon steel sheet, preferably, crystal grains having a crystallographic orientation difference within .+-.15 degrees from a cubic orientation of {100}&lt;001&gt; occupy an areal percentage of not less than 70%, or the thickness of a surface oxide layer of the steel sheet is not greater than 0.5 .mu.m. The method for manufacturing a doubly oriented magnetic steel sheet as mentioned above in (2) includes the steps of hot-rolling and cold-rolling steel containing C in an amount of 0.02% to 0.2% and Si and M in amounts satisfying a predetermined formula of relation, wherein annealing is performed at a temperature not lower than 750.degree. C. and through quick application of heat during cold rolling; and the obtained steel sheet is annealed under reduced pressure through use of an annealing separator. In this method for manufacturing a doubly oriented magnetic steel sheet, preferably, a rolling reduction is 40% to 85% in cold rolling performed before and after intermediate annealing.

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Osaka, Japan

Toshiro Tomida